To set the value of a two-dimensional array in c++ dll

Ken H

11y3kVisual Basic .NET

Hello friend,

The following code will not run properly.

C++ codes:

// mytest.cpp

#include "stdafx.h"

#include

extern "C"{

_declspec(dllexport) void _setv(char ***s, int row, int column){

for (int i = 0; i < row;i++){

for (int j = 0; j < column;j++)

{

strcat(s[i][j],"The value in here.");

}

C# codes:

class Program

{

[DllImport("mytest.dll", CallingConvention = CallingConvention.Cdecl)]

public static extern void _setv(string[,] s, int row, int column);

static void Main(string[] args)

{

string[,] _arr = new string[2, 2]{{"",""},{"",""}};

_setv(_arr, 2, 2);

}

Thank.

4 Replies

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Vulpes11y ago
Accepted answer
The P/Invoke marshaler seems to have problems with a 2D string array and the only way I could get this to work was to 'flatten' it into a single dimensional string array and then pass that by reference.

There are a couple of other points to note:

1. As the unmanaged function is filling the array and then returning it to managed code, a large enough buffer needs to be created for each string element including the terminating null character - I've used 80 characters.

2. Normally, you'd use StringBuilder instead of string for a mutable string but an array of StringBuilders doesn't work and therefore you have to use a string array. However, you need to specify the [In, Out] attributes for this to work.

3. The marshaler needs to know how big the string array is and I've specified 4 as there are 2 x 2 elements. The 'row' parameter has been changed to 'size' to reflect this.

So, here's the revised code:

// C++ code

// mytest.cpp
#include "stdafx.h"
#include
#include

extern "C"
{
_declspec(dllexport) void _setv(char ***s, int size, int column)
{
int row = size/column;
char **t = *s;

for (int i = 0; i < row;i++)
{
for (int j = 0; j < column;j++)
{
strcpy(t[i * row + j], "The value in here.");
}
}
}
}

// C# code

using System;
using System.Runtime.InteropServices;

class Program
{

[DllImport("mytest.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern void _setv([In, Out][MarshalAs(UnmanagedType.LPArray,
ArraySubType= UnmanagedType.LPStr, SizeParamIndex=1)] ref string[] s, int size, int column);

static void Main(string[] args)
{

string[] s = new string[4];
for(int i = 0; i < 2; i++)
for(int j = 0; j < 2; j++)
{
s[i * 2 + j] = new string('\0', 80);
}
_setv(ref s, 4, 2);

string[,] ss = new string[2, 2];

for(int i = 0; i < 2; i++)
for(int j = 0; j < 2; j++)
{
ss[i, j] = s[i * 2 + j];
Console.WriteLine(ss[i, j]);
}

Console.ReadKey();
}
}

As expected, the output is:

The value in here.
The value in here.
The value in here.
The value in here.
Ken H11y ago
Thank for Vulpes. :)
Vulpes11y ago
To deal with unicode, as opposed to ANSI, strings you'll need to make a few changes:

// C++ code

// mytest_uni.cpp
#include "stdafx.h"
#include
#include

extern "C"
{
_declspec(dllexport) void _setv(wchar_t ***s, int size, int column)
{
int row = size/column;
wchar_t **t = *s;

for (int i = 0; i < row;i++)
{
for (int j = 0; j < column;j++)
{
wcscpy(t[i * row + j], L"The value in here.");
}
}
}
}

// C# code

using System;
using System.Runtime.InteropServices;

class Program
{

[DllImport("mytest_uni.dll", CharSet = CharSet.Unicode, CallingConvention = CallingConvention.Cdecl)]
public static extern void _setv([In, Out][MarshalAs(UnmanagedType.LPArray,
ArraySubType= UnmanagedType.LPWStr, SizeParamIndex=1)] ref string[] s, int size, int column);

static void Main(string[] args)
{

string[] s = new string[4];
for(int i = 0; i < 2; i++)
for(int j = 0; j < 2; j++)
{
s[i * 2 + j] = new string('\0', 80);
}
_setv(ref s, 4, 2);

string[,] ss = new string[2, 2];

for(int i = 0; i < 2; i++)
for(int j = 0; j < 2; j++)
{
ss[i, j] = s[i * 2 + j];
Console.WriteLine(ss[i, j]);
}

Console.ReadKey();
}
}
Ken H11y ago
It can work. but there is a small problem, and how to make it compatible with Unicode character set?

Thank for you Vulpes. :)